A mechanistic study of allene carboxylation with CO2 resulting in the development of a Pd(II) pincer complex for the catalytic hydroboration of CO2

The carboxylation of allenes with CO2 represents a potentially important method for the synthesis of unsaturated carboxylic acids. Here, we describe a detailed mechanistic study of the catalytic carboxylation of allenes using (PSiP)-P-Cy ((PSiP)-P-Cy = Si(Me)(2-PCy2-C6H4)(2)) supported Pd complexes. As part of this work we have identified, characterized and isolated all of the proposed intermediates in the catalytic cycle and shown that they are kinetically competent catalysts. In addition, we have isolated several off-cycle species, which are in equilibrium with complexes in the catalytic cycle, and established that they are also active catalysts. Several of these off-cycle species are formed through an unusual ligand rearrangement of the (PSiP)-P-Cy pincer ligand, in which a Si-C bond is reversibly cleaved. The major catalyst deactivation pathway has been identified. Furthermore, our mechanistic study has allowed us to develop a new catalyst for the hydroboration of carbon dioxide, which gives a maximum turnover number (TON) greater than 60 000; the highest reported to date.